1. Field of the Invention
The present invention relates generally to transponders and more particularly to satellite transponders.
2. Description of the Related Art
Transponders are widely used in a variety of communication systems, e.g., the satellite communication system 20 of FIG. 1. In this communication system, a plurality of satellites 22, 24, 26, 28, 30 and 32 are stationed in a geostationary orbit (GEO) 34 about the Earth 36. These satellites and coverage areas on the Earth 36 can all be interconnected by communication links (a coverage area is that portion of the Earth's surface that is intersected by a satellite antenna beam). In particular, the satellite 24 is shown to have a communication link 40 with a coverage area 41 on the Earth 36 and communication links 42 and 46 with satellites 22 and 26 respectively.
Such communication systems are useful in various communication interconnections. For example, the system 20 can include a communication link 48 to the satellite 30 and a communication link 50 from that satellite to an out-of-sight coverage area on the Earth. This extended system can couple communications between the coverage area 41 and the out-of-sight coverage area. As a second example, the system can also include a communication link 52 to a reconnaissance satellite 54 that is in a low Earth orbit 56. The satellite 54 gathers data through a reconnaissance link 57 and transfers that data on a real-time basis to the out-of-sight coverage area through the system 20.
Typically, each of the communication links has a plurality of frequency channels. Communication versatility is enhanced if the satellite 24 can receive a signal in a frequency channel of one communication link, e.g., link 40, and route the signal so that it is transmitted on a selected frequency channel of another communication link, e.g., link 46.
FIG. 2 is an enlarged view of the satellite 24 with its communication links 40, 42 and 46 coupled respectively through antennas 60, 62 and 66. Each of these antennas can be used for receiving and transmitting signals, e.g., with different polarizations, from different satellites or coverage areas. Alternatively, each antenna symbol may represent a pair of receive and transmit antennas.
The latter arrangement is illustrated in FIG. 3 in which the receive antennas are referenced as 60R, 62R and 66R and the transmit antennas as 60T, 62T and 66T. As schematically indicated, each of the receive antennas receives signals in n.sub.i input frequency channels 67 and each of the transmit antennas transmits signals in no output frequency channels 69. The frequency channels are shown in ascending frequency order, e.g., n.sub.i is the highest frequency input channel.
Three exemplary routing paths 70, 72 and 74 are shown. Signal 70 is routed from input frequency channel 2 at receive antenna 60R to output frequency channel 1 at transmit antenna 62T. Signal 72 is routed from input frequency channel 3 at receive antenna 62R to output frequency channel 2 at transmit antenna 60T. Signal 74 is routed from input frequency channel n.sub.i at receive antenna 66R to output frequency channel 3 at transmit antenna 62T. At another instant in time, the signals 70, 72 and 74 may be routed to different output frequency channels. Preferably, a signal from any of the n.sub.i input frequency channels can be routed to any selected one of the n.sub.o output frequency channels.
A conventional transponder structure for performing this function is the crossbar switch 80 of FIG. 4. As defined in various references (e.g., Modern Dictionary of Electronics, Graf, Rudolf F., SAMS, Carmel, Ind., sixth edition, 1991, p 213) and referenced in FIG. 4, a crossbar switch is a switching system having a plurality of input paths 82, a plurality of output paths 84 and a matrix of switches 86 (e.g., electromagnetically or electronically operated switches) for interconnecting any one of the input paths with any one of the output paths. The input paths 82 and output paths 84 are generally connected to input and output ports of the crossbar switch.
The communication links of FIGS. 1 and 2 are typically microwave links, e.g., X band and Ku band. If the crossbar switch 80 were used to route signals in the satellite 24 of FIG. 3, the signals would preferably be processed in an intermediate frequency band at which the switches 86 can be more easily and economically fabricated. In this satellite configuration, downconversion and upconversion operations would be associated respectively with the receive antennas and the transmit antennas.
Although the crossbar switch 80 could perform the routing operations of FIG. 3, its structure would be complex and expensive. In general, if the satellite 24 has n.sub.i input frequency channels at each of m.sub.i receive antennas and n.sub.o output frequency channels at each of m.sub.o transmit antennas, the crossbar switch 80 would require m.sub.i n.sub.i input paths 82, m.sub.o n.sub.o output paths 84 and (m.sub.i n.sub.i)(m.sub.o n.sub.o) switches 86. For example, if m.sub.i =m.sub.o =3 and n.sub.i =n.sub.o =50, the crossbar switch 80 would have 22,500 of the switches 86.
Another transponder system was disclosed in copending U.S. patent application Ser. No. 08/334,491 which was filed Nov. 3, 1994 in the name of Copeland, Wilbert B., et al. and assigned to Hughes Electronics, the assignee of the present invention. This system includes a plurality of channel control units (CCU) which downconvert a band of signals to an intermediate frequency for efficient filtering and separation, and then, subsequently, individually reconvert the signals back to the output microwave band. This configuration did not originally envision any rearrangement or recombination of the signals within the common bandwidth.